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Herring Gulls Larus argentatus in Common Eider Somateria mollissima feeding flocks – a discerning kleptoparasite

CHRIS M. WALTHO

73 Stewart Street, Carluke, Lanarkshire, Scotland, UK. E-mail: [email protected]

Abstract In Scotland, feeding flocks of Common Eider Somateria mollissima are regularly attended by Herring Gulls Larus argentatus which attack and steal food from the eiders. This study describes the prey brought to the surface by the eiders, their prey handling behaviour, subsequent attacks by gulls, and the outcome of such attacks. The majority of the prey was Blue Mussel Mytilus edulis (80%) but Green Urchin Psammechinus miliaris, Shore Crab Carcinus maenas and Common Starfish Asterias rubens were also taken. Attack frequency was not related to the occurrence of each type of prey in the eiders’ diet, except that gulls were only observed attacking eiders for non- mussel prey. Attacks were most frequent (79%), and most successful (i.e. losses to gulls were highest), against eiders with starfish (73% of prey lost) and crabs (50% of prey lost). In contrast, losses of urchins (4%) and mussels (0%) were low. Despite starfish accounting for < 4% of Common Eider prey, gulls won sufficient (73%) to make kleptoparasitism a common feature of feeding flocks of eiders in this study area. These results indicate that Herring Gulls are highly selective in their attacks on Common Eider, and in the prey that are targeted.

Key words: Common Eider, Herring Gull, kleptoparasitism, Larus argentatus, Somateria mollissima.

Kleptoparasitism involves stealing by one 3) large, high-quality food items, 4) individual (the kleptoparasite) food already predictable food supply, 5) visible food with procured by another individual (the host; the host, 6) food shortage for kleptoparasite, Brockmann & Barnard 1979). A number and 7) prey handling time by the host. of interspecific associations have been Where diving waterfowl regularly bring large identified (Brockmann & Barnard 1979) prey to the surface, and especially where as potentially facilitating the development these are handled before ingestion, the prey of kleptoparasitism: 1) large host are temporarily available to a potential concentrations, 2) large quantities of food, kleptoparasite (Amat 1990). Where the

©Wildfowl & Wetlands Trust Wildfowl (2009) 59: 41–52 42 Herring Gulls kleptoparasitising Common Eider kleptoparasite feeds at shallower depths Herring Gulls kleptoparastising eiders for than the host species it gains access to prey urchins, but intriguingly not for mussels. that it cannot reach itself. Amat (1990) also Hence, despite its widely reported nature, identified a number of possible evasion there is little empirical evidence for the type tactics used by hosts to reduce the risk of prey that the Herring Gulls are actually from the kleptoparasite, as follows: 1) stealing from the eiders. This study aims to synchronisation of feeding activities within record the prey brought to the surface by the feeding flock to confuse kleptoparasites, Common Eider, their prey handling 2) handle prey more rapidly, 3) increase their behaviour, subsequent attacks by gulls, and distance from the kleptoparasite, 4) shift to the outcome of such attacks. a less vulnerable diet, and 5) keep the prey out of the kleptoparasite’s sight. Methods Brockmann and Barnard (1979) have Gare Loch (56°01’N 04°47’W) and Loch recorded Common Eider Somateria mollissima Long (56°01’N 04°52’W) are two adjoining being kleptoparasitised by several large gull sheltered sea lochs 40 km northwest of species, notably Herring Gull Larus Glasgow, within the Firth of Clyde, west argentatus, Lesser Black-backed Gull Larus Scotland. These sea lochs have a contiguous fuscus, Great Black-backed Gull Larus marinus coarse sediment (cobble and boulder) and Glaucous Gull Larus hyperboreus. In a coastline. The littoral and infralittoral (upper study of the kleptoparasitic behaviour of sublittoral) zones are dominated by Blue Glaucous Gulls taking food from eiders in Mussel Mytilus edulis beds, together with southwest Iceland, Ingolfsson (1969) noted Sugar Kelp Laminaria saccharina grazed by that the gulls defended feeding flocks of Green Urchin Psammechinus miliaris in the eiders against other gulls, including Great infralittoral zone. The coast road around Black-backed Gulls. This has also been Gare Loch has frequent viewing points; the witnessed in Scotland (Prys-Jones 1973). southeast shore of Loch Long also has a Neither Ingolfsson (pers. comm.) nor coastal road with similar viewing Prys-Jones (1973) identified the prey stolen opportunities. by the gulls but presumed them to be Observations were made on 34 days mussels. Kleptoparasitising of eiders by over 22 months between July 2000 and Herring Gulls is well known (Amat 1990; March 2003, from a parked vehicle at a Brockmann & Bernard 1979; Kallander range of 50–300 m from flocks of feeding 2006). Kallander (2006) found the eiders, using 10 × 40 binoculars and a 30 × association between Herring Gulls and 80 telescope. Feeding flocks were first Common Eiders to be one of the most counted to determine flock size and the frequent kleptoparasitic relationships that numbers of Herring Gulls present within 10 he observed in southern Sweden, but was m of the flock were also counted. The flock not specific about the prey that the gulls was then systematically scanned from left to were stealing from the eiders. In eastern right, during which time all prey brought to Canada, MacCharles (1997) recorded the surface for handling was identified using

©Wildfowl & Wetlands Trust Wildfowl (2009) 59: 41–52 Herring Gulls kleptoparasitising Common Eider 43 the techniques described below. Any attacks mussels vigorously, breaking the byssal by gulls and the outcome of the encounter threads to separate individuals. As the bird were recorded. The scan continued until grasps a preferred mussel, others separated feeding in the flock had ceased and in this way are not subsequently retrieved birds began to drift offshore. Although from the water. With larger mussels, this is this technique may be subject to frequently followed by a distinct head back pseudoreplication due to the potential for motion to swallow the mussel whole. On repeat sampling of individual birds within a bringing Common Starfish to the surface, feeding flock in a single feeding bout, each eiders repeatedly and rapidly open and surfacing represents a separate event. closed the bill to manoeuvre all starfish Attendant gulls would not have been able to limbs into their mouth. Birds sometimes predict which prey item would be brought to hold the starfish on the surface of water to the surface by an individual eider, and so help capture all the limbs before swallowing prey availability to the gulls was effectively whole. Handling of Green Urchin is random. Attacks by Herring Gulls could characterised by the eiders rolling urchins result in three potential outcomes: prey won around their bills, on their broader axis, to by the gull, prey won by the eider and prey flatten or break off spines (MacCharles lost to both. Eiders not subject to attack 1997). The birds also regularly dip their bill were considered to have successfully (and urchin) into the water to help the handled and ingested the prey. Attack rates rolling action before swallowing the urchin were expressed as a percentage of each of whole. On taking Shore Crab Carcinus the prey species being made available to maenas, the eiders shake the crabs vigorously gulls by the feeding eider bringing them to to disarticulate and de-limb them (Picozzi the surface for handling. Loss rates to gulls 1958), often necessitating retrieval of the were calculated for each type of prey as the body lost back into the water. In contrast to proportion of Herring Gulls attacks that mussels, the crab body was always retrieved resulted in the eiders losing their food to the from the water and the process repeated gulls. Overall loss rates included the prey until all limbs had been removed, when the won by gulls and those lost by both gull and body was swallowed whole. These prey eider. The retention rate includes the prey handling techniques are distinctive and not subject to attack and those won by readily observable, through a telescope, in eiders. flocks feeding at distances of up to ~ 300 m. Eiders handle different prey in different Prey handling times were determined ways. Feeding techniques and associated from additional observations (outside of behavioural traits have been noted in some flock scans) for a small number of cases detail and are summarised as follows. When where the birds were feeding on mussels feeding on Blue Mussel, surfacing eiders (n = 62), urchins (n = 87), and very often bring up mussels in clumps, the occasionally starfish (n = 4). The small mussels being attached to each other by sample size for Common Starfish was due to strong byssal threads. Birds shake the their low frequency and high kleptoparasitic

©Wildfowl & Wetlands Trust Wildfowl (2009) 59: 41–52 44 Herring Gulls kleptoparasitising Common Eider loss rate. Handling time was not recorded the number of attendant Herring Gulls and for eiders feeding on crabs because there the number of birds in the feeding flocks of were very few such cases seen during the eiders observed during the study (linear 2 survey. regression: y = 0.056× + 0.707, R 121= Linear regression analysis determined 0.713, P < 0.01, Fig. 1). there was a significant association between A total of 5,346 prey items were Common Eider flock size and the number identified in the 152 scans of feeding flocks of Herring Gulls in attendance, and chi- in the study area: 4,262 (79.7%) mussels, 875 square tests were used to test the null (16.4%) urchins, 199 (3.7%) starfish and 10 hypothesis that there were no differences in (0.2%) crab (Fig. 2). Prey handling times Herring Gull attack rate, loss rate or were determined separately for 153 items of retention rate across prey species. A z-test three prey species (Fig. 3). There was no (Fowler & Cohen 1988) compared the mean difference in handling times recorded for handling times recorded for eiders feeding mussels (mean = 13.29 s, s.e. ± 1.40, range on different species. Herring Gull prey = 2–56) and urchin (mean = 16.42 s, s.e. ± z selectivity was assessed using the Linear 1.54, range = 2–75) ( 61,86 = 1.50, n.s.). Index of Prey Selection (Strauss 1979). The Although eiders were timed handling and index is: Li = dl – hl, where dl is the ingesting starfish on only four occasions, the proportional density of prey item i in the handling time for this type of prey (mean = diet (Herring Gull attacks) and hl the 29.0 s, s.e. ± 4.45, range = 16–36) was proportional density available (i.e. numbers significantly greater than for both mussels z brought to the surface by eiders). It ranges and urchins (mussel versus Starfish, 61,4 = ≤ z from –1 to +1, with positive values 3.36, P 0.001; urchin versus starfish 86,4 = indicating selection and negative values 2.66, P ≤ 0.001). indicating either avoidance by the predator During the 152 observations of feeding or reduced prey accessibility. The expected flocks, 123 (81%) flocks had attendant value for random feeding is zero. Herring Gull, while 29 (19%) did not. Gull attacks were observed during 85 (56%) of Results the flock observations and a total of 213 individual gull attacks were recorded. The A total of 152 scans of eider feeding flocks proportion of prey attacked by Herring were made at 19 separate locations around Gulls was not consistent across prey species χ2 the two lochs (Table 1). On 29 occasions ( 3 = 3015, P < 0.001). Although mussels there were no gulls in attendance. The mean were the most frequent prey (79.7%), there number of Common Eider in the study area were no attacks by gulls on eiders handling was 1,711 birds (s.e. ± 198, range = mussels seen throughout the study. Gull 674–3,565, n = 22 monthly counts), with the attacks were most frequent when the eiders birds occurring in flocks of varying sizes were feeding on starfish (3.7% of prey, (mean = 16.99, s.e. ± 1.38, range = 2–114). 79.4% of attacks) and urchins (16.4% of There was a positive relationship between prey, 5.6% of attacks; Fig. 4). The Linear

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Table 1. Data on prey taken by Common Eiders and kleptoparasitic attacks by Herring Gulls recorded during observations made at Gare Loch and Loch Long, Firth of Clyde, Scotland from July 2000–February 2003.

Observation point No. No. No. No. No. No. flock observation individual mussels other klepto- scans days feeding recorded prey parasitic events recorded attacks

Kidston Point 1 1 40 37 3 2 Royal Northern Yacht 13 10 659 538 121 26 Club Blairvadach 14 6 319 228 91 14 Shandon Old Church 5 3 141 85 56 7 Shandon/Faslane S 15 9 697 618 79 27 Garelochhead Bay 7 4 131 131 0 0 Dalandhui Point 5 2 25 20 5 1 Rahane 2 1 115 110 5 1 Little Rahane 10 9 833 800 33 17 Crossowen Pt Clynder 26 13 730 557 173 18 Clynder Post Office 3 2 155 135 20 6 Stroul Bay 13 9 426 399 27 16 Camsail Bay 1 1 35 25 10 6 Kilcreggan East 8 4 115 34 81 18 Kilcreggan West 2 1 25 0 25 3 Cove Bay 7 4 124 24 100 15 Knockderry Point 2 1 52 0 52 2 Ardpeaton 3 2 106 101 5 0 Coulport Bay 15 9 618 420 198 34

Total 152 34 5,346 4,262 1,084 213

Index of Prey Selection indicates that the random (Table 2). Gulls were successful in Herring Gulls selected starfish (0.70) and 69% of all attacks. Most successful were avoided mussels (–0.80), whereas feeding on attacks for starfish (73%, 115 won, 43 lost), urchins (0.07) and crabs (0.03) was close to followed by crabs (67%, 4 won, 2 lost) and

©Wildfowl & Wetlands Trust Wildfowl (2009) 59: 41–52 46 Herring Gulls kleptoparasitising Common Eider Numbers of Herring Gulls

Numbers of Common Eiders

Figure 1. Numbers of Herring Gulls present, in relation to Common Eider flock sizes (n = 123 flocks). See text for statistics. % Prey

Mussel Urchin Starfish Crab Prey Species

Figure 2. Percentage of different types of prey brought to the surface by Common Eider (n = 5,346 feeding events). urchins (57%, 28 won, 21 lost) (Fig. 5). therefore retained all of their prey. Those Starfish were significantly more likely to be Eiders feeding on urchins, despite gull won by Herring Gulls from Eiders than attacks, retained 96% of them (Table 2, Fig. χ 2 urchins ( 1 = 3.98, P < 0.05). Eiders 5). Eiders feeding either on mussels or on feeding on mussels were not attacked and urchins kept virtually all of their prey, and

©Wildfowl & Wetlands Trust Wildfowl (2009) 59: 41–52 Herring Gulls kleptoparasitising Common Eider 47 Prey handling time(s)

Mussel Urchin Starfish

Figure 3. Mean prey handling time (± 95% confidence intervals) for Common Eiders observed in west Scotland (n = 62 for mussels, n = 87 for urchins and n = 4 for starfish).

Starfish

Crab

Mussel % Kleptoparasitic attack by gulls by % Kleptoparasitic attack

% Prey availability from Common Eiders

Figure 4. Percentage of kleptoparasitic attacks by Herring Gulls in relation to the type of prey brought to the surface by Common Eiders. the difference in prey conservation kleptoparasitism was not consistent across χ2 χ2 efficiency was not significant ( 1 = 0.07, prey species ( 3 = 2496, P < 0.001). Those n.s.). Crab feeders retained 50%. The feeding on starfish retained only 36% of proportion of prey retained by Eider prey brought to the surface. For starfish, this compared to that lost due to difference between prey retained by eiders

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Table 2. Frequency and outcome of kleptoparasitic attacks by Herring Gulls on Common Eiders feeding on different types of prey item.

Starfish Urchin Crab Mussel

Prey brought to surface by Common Eider 199 875 10 4,262 Kleptoparasitic attack by Herring Gull 158 49 6 0 Kleptoparasitic attack rate % 79.4 5.6 60.0 0 Linear Index of Prey Selection 0.70 0.07 0.03 –0.80

Outcome of attacks recorded (n) 158 49 6 0 % won by Herring Gull (n) 72.8 (115) 57.1 (28) 66.7 (4) 0 % won by Common Eider (n) 19.6 (31) 32.7 (16) 16.7 (1) 0 % lost to both (n) 7.6 (12) 10.2 (5) 16.7 (1) 0 % Common Eider prey retention rate (n) 36.2 (72) 96.2 (842) 50.0 (5) 100.0 (4,262)

and that lost due to kleptoparasitism, 1973; Kallander 2006). Yet although compared with eiders feeding on mussels kleptoparasitism by Herring Gulls was and urchins, was significant (mussels versus evident in the present study, the gulls were χ2 starfish, 1 = 2778, P < 0.001; urchin not observed taking mussels from the eiders, χ2 versus starfish, 1 = 459, P < 0.001). despite it being the most common prey. The Herring Gulls’ success rates observed in this Discussion study on starfish (73%), crabs (67%) and urchins (57%) were much greater that the This study found that Herring Gulls are 38.5% (n = 325) recorded by Kallander frequently present in feeding flocks (2006). of Common Eiders, where they The ecological conditions proposed kleptoparasitise the eiders’ prey in a non- by Brockmann & Barnard (1979) as random manner. Specifically, the Herring potentially facilitating the development of Gulls were highly selective for starfish but kleptoparasitism may help to explain the not for mussels. Mussels form a major differing results. Eiders generally feed in component of the Common Eider’s diet flocks which are variable in size, and flock across most of its geographic range, and size is likely to be influenced by the size and have been considered to be the target quality of food patches (Guillemette & of kleptoparasitism by Herring Gulls Himmelman 1996). The present study elsewhere, including in other parts of found that the numbers of associating gulls Scotland (Ingolfsson 1969; Prys-Jones increased significantly with the size of the

©Wildfowl & Wetlands Trust Wildfowl (2009) 59: 41–52 Herring Gulls kleptoparasitising Common Eider 49 Outcome of attack (%)

Mussel Urchin Starfish Crab Prey item

% prey retained by Common Eider % prey won by Herring Gull % prey lost to both

Figure 5. Outcome of kleptoparasitism by Herring Gulls on Common Eiders. eider flocks. Eiders feed principally over that Eider brought to the surface in the mussel beds where there can be a plentiful presence of gulls may suggest that the host supply of food, but they also exploit higher may already adjust their dietary choice when quality food items such as starfish and crabs, selecting prey on the bottom. Energy which are visible and attractive to Herring content of starfish (23.3 KJ g–1 ash-free dry Gulls. These more profitable food items weight, Larsen & Guillemette 2000) and also require prolonged handling times by the crabs (23.7 KJ g–1 AFDW, Klein Breteler eiders, extending their availability to 1975) is higher than that of mussels (19.5 KJ kleptoparasitic gulls. All of these food items g–1 AFDW, Elner & Hughes 1978) or are accessible to gulls only at the lowest part urchins (17.4 KJ g–1 AFDW, Otero- of the tide cycle, and so are largely Villanueva et al. 2004). Starfish and crabs, as unavailable in the absence of kleptoparasitic predators, occupy a higher trophic level behaviour. in the ecosystem, than either urchins Possible evasion tactics (Amat 1990) are (omnivores) or mussels (filter-feeders). used by eiders to reduce the risk from the Other things being equal, the energy gained kleptoparasitic Herring Gull. Synchronised by both Common Eiders and Herring Gulls feeding, diving and surfacing has been feeding on starfish and crabs would described in eider feeding flocks subject therefore be expected to be greater than on to kleptoparasitic behaviour by Herring urchins or mussels. However, extracting the Gulls elsewhere (MacCharles 1997). The flesh content from the shell/exoskeleton dominance of lower energy content prey comes at a cost. Bustnes (1998) has shown (namely mussels and urchins) in the items that eiders select mussel size to minimise

©Wildfowl & Wetlands Trust Wildfowl (2009) 59: 41–52 50 Herring Gulls kleptoparasitising Common Eider shell content. Similarly, there are costs for urchins are taken to the shore by the gulls for the Herring Gull in handling and processing handling and ingestion. the captured prey. The smallest prey can Although not evident in the present often be swallowed in situ but larger prey study, other research has shown that requires more handling effort. Prey that has Herring Gulls do take mussels of varying to be broken open by dropping on to, sizes (Hilgerloh et al. 1997). Smaller ones will or hammered against, a hard surface be swallowed whole, and the shell remains necessitates the gull to leave the feeding subsequently regurgitated as a pellet. Larger eider flock and fly to the shore. mussels are taken to a hard surface where Starfish kleptoparasitised from eiders are they are repeatedly dropped from a height to often ingested in situ without the gulls break into them. having to leave the eider feeding flock. Of the prey that Herring Gulls However, gulls with larger starfish are often kleptoparasitise from eiders, starfish appear pursued by other gulls, showing intra- to be the easier for the gulls to handle and specific kleptoparasitism, and forced to ingest. Processing starfish can take place leave the eider feeding flock to retain their while the gull remains within the eider prey. Verbeek (1977a, b) found that, when feeding flock, increasing opportunities for available, starfish are clearly preferred by further attack attempts during the remainder Herring Gulls over mussels, but they are of the eider flock feeding bout. only normally available to Herring Gulls at Kleptoparasitic prey selection by Herring the lowest stages of the tidal cycle period, Gulls is likely to be determined by a balance which is short (about 2 h) and infrequent of prey energy value gained set against prey (only during spring tides). handling and processing costs. When Small crabs kleptoparasitised from compared to the other prey made available eiders can be ingested whole in situ, but by the feeding eider, the predominance of larger ones have to be taken to the shore to starfish selected by Herring Gulls found in be handled, where they are either repeatedly this study can be attributed to either, or a dropped or turned over and the underside combination of, higher energy value and hammered open by the bill. lower processing costs. In eastern Canada, MacCharles (1997) Given the very low proportion of crabs observed that eiders fed preferentially on in the overall prey spectrum, the mussels, and that Green Urchins were kleptoparasitic association between Herring also an important source of food, but Gulls and Common Eiders appears to kleptoparasitic attacks by Herring Gulls were depend solely upon the eiders continuing to recorded only when the eiders were feeding bring starfish to the surface. The reward to on urchins. She described gulls rushing at an the gulls from pursuing urchins would likely eider as soon as it surfaced, to try to steal be insufficient to support kleptoparasitism urchins. Urchins are only normally available as a viable feeding strategy. There remains a to gulls at the lowest stages of the tidal cycle. question about the persistence of starfish in As for the crabs, all except the smallest eiders’ diet, given the high rate of gull

©Wildfowl & Wetlands Trust Wildfowl (2009) 59: 41–52 Herring Gulls kleptoparasitising Common Eider 51 attacks and low retention rate by the ducks. Fowler, J. & Cohen, L. 1988. Statistics for There may be more to starfish than their Ornithologists. BTO Guide No. 22. British energy value alone implies. There is a need to Trust for Ornithology, Thetford, Norfolk. examine the relative roles of: 1) minimising Guillemette, M. & Himmelman, J.H. 1996. shell or exoskeleton intake and their Distribution of wintering common eiders processing costs, 2) salt water intake and the over mussel beds: does the ideal free Oikos cost of salt excretion, 3) the range and distribution apply? 76: 435–442. amount of various carotenoids in the diet, Hilgerloh, G., Herlyn, M. & Michaelis, H. 1997. and 4) the possible parasite loading for each The influence of predation by herring of the Common Eider prey species. These all gulls Larus argentatus and oystercatchers Haematopus ostralegus on a newly established remain to be assessed in the context of mussel Mytilus edulis bed in autumn and kleptoparasitism of eiders by Herring Gulls. winter. Helgoland Marine Research 51: 173– Additionally, there is a need to test for 189. any differences in the feeding behaviour of Ingolfsson, A. 1969. Behaviour of gulls robbing the Common Eider with and without eiders. Bird Study 16: 45–52. kleptoparasitising gulls being present. Klein Breteler, W.M. 1975. Food consumption, Acknowledgements growth, and energy metabolism of juvenile shore crabs, Carcinus maenas. Netherlands I wish to thank Agnar Ingolfsson for his useful e- Journal of Sea Research 9: 255–272. mail discussions during the early phases of this study. Thanks also to Tony Fox for his Larsen, J.K. & Guillemette, M. 2000. Influence of encouragement and for improving an earlier annual variation in food supply on version of this manuscript. I also wish to thank abundance of wintering common eiders Matthieu Guillemain, Eileen Rees and two Somateria mollissima. Marine Ecology Progress anonymous referees for their guidance and Series 201: 301–309. patience. Kallander, H. 2006. Interspecific kleptoparasitism by four species of gull Larus spp. in South References Sweden. Ornis Svecica 16: 127–149. Amat, J. A. 1990. Food usurption by waterfowl MacCharles, A.M. 1997. Diving and foraging and waders. Wildfowl 41: 107–116. behaviour of wintering Common Eider Brockmann, H.J. & Bernard, C.J. 1979. Somateria mollissima at Cape St Mary’s Kleptoparastism in birds. Animal Behaviour Newfoundland. M.Sc. Thesis, Simon Fraser 27: 487–514. University, Vancouver, Canada.

Bustnes, J.O. 1998. Selection of blue mussels, Otero-Villanueva, M.M., Kelly, M.S. & Burnell Mytilus edulis, by common eiders, Somateria G. 2004. How diet influences energy mollissima, by size in relation to shell content. partitioning in the regular echinoid Canadian Journal of Zoology 76: 1787–1790. Psammechinus miliaris; constructing an energy budget. Journal of Experimental Marine Biology Elner, R.W. & Hughes, R.N. 1978. Energy and Ecology 304: 159–181. maximization in the diet of the shore crab, Carcinus maenas. Journal of Animal Ecology. 47: Picozzi, N. 1958. Eider’s method of eating crabs. 103–116. British Birds 51: 308.

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Prys-Jones, O.E. 1973. Interactions between gull Verbeek, N.A.M. 1977a. Comparative feeding and eiders in St Andrew’s Bay, Fife. Bird Study ecology of Herring Gulls Larus argentatus and 20: 311–313. Lesser Black-backed Gulls Larus fuscus. Ardea 65: 25–42. Strauss, R.E. 1979. Reliability estimates for Ivlev’s electivity index, the forage ratio, and a Verbeek, N.A.M. 1977b. Comparative feeding proposed linear index of food selection. behaviour of immature and adult Herring Transactions of the American Fisheries Society 108: Gulls. The Wilson Bulletin 89: 415–421. 344–352.

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